You've made some dire mistakes here.

I'm a damn good driver, but I have next to no idea how cars work. I can just barely change a tire. I don't need to know how it works to know how to use it.

Under normal conditions, sure. But if you find yourself on ice, or hydroplaning, or driving through Death Valley, or running on empty while stuck in traffic, a basis of understanding of the physics of automobiles will absolutely help you out. By willfully ignoring the basic function of the automobile you are permitting yourself to be a victim of circumstance.

I have only the faintest of notions of what a past participle is and I don't think it really matters. Unless I'm trying to teach English I don't need to know it. We don't learn language by learning what the parts are, we learn it by using it and by hearing others use it. People are using adverbs correctly before they have any idea what an adverb is. You don't learn what nouns are before you learn what cats, dogs, and balls are.

But no one benefits by shrouding the language arts in gnostic mystery. Learning to diagram a sentence is the best way to learn the craft of building sentences. I'd have to look up a participle phrase, too - but I learned them, and I know I can learn them again. I don't remember my trigonometric identities either but if I have to walk through them again, I can. A basis of understanding is always better than a basis of mystery.

There's nothing wrong with ending a sentence with a preposition and 'ain't' is totally a word.

Ain't is a word used in certain registers but not all. Using "ain't" in a technical report is inappropriate. Using technical language in colloquial speech is inappropriate. The art of learning the appropriate use of language is the art of learning language.

You might want to be able to distinguish between 'their' and 'they're' if you want to come across as vaguely intelligent, but it's honestly not that important to most people's interactions.

Au contraire. You mix up "their" and "they're" and I know you're either (A) stupider than me (B) don't care enough to address me with respect. Someone with precise language will automatically think less of someone with imprecise language and your argument for "good enough" illustrates that you don't value treating our conversations with care. That will not benefit you.

I mean, sure, being well spoken can make you seem intelligent, but it's not as good as having something significant to say, and outside of a classroom context correcting other people can be worse than not saying anything at all.

But if you can't say it according to the agreed-upon ground rules of written or spoken communication, no one will listen to you.

There are social signals involved in language. You advocate ignoring them because they aren't important to you. They aren't important to you because you never learned their importance, which means you are operating at a disadvantage.

I'n'I can throw down in any f'n register I wanna use. It ain't no thang. A skilled interlocutor can converse in the appropriate register, regardless of his upbringing or background. Conversational versatility maximizes your rhetorical prowess because your statement isn't obscured by syntax. 'n I can be completely full of shit and you'd never know 'cuz your head so fulla pride you can't fuckin' hear me, son.

Ever studied cockney?

It's a deliberate obfuscation of language to build affinity amongst a disadvantaged socioeconomic segment. So if you speak Cockney, you can reach them. If you speak the Queen's English, you will forever be an outsider.

That is the study of language - knowing how to make yourself heard. You seem to think that if you shout loud enough it'll work out. It won't.